1. Field of the Invention
The present invention relates to novel zeolites based on silica and titanium oxide and to a process for the preparation thereof; more especially this invention relates to the production of MFI zeolites.
2. Description of the Prior Art
Zeolites are crystallized tectosilicates. Their structures consist of conglomerations of TO.sub.4 tetrahedrons defining a tridimensional skeleton via the sharing of oxygen atoms. In zeolites of the aluminosilicate type, which are the most common, T represents the tetravalent silicon, together with the trivalent aluminum. The cavities and channels of molecular dimensions of this skeleton accept cations to compensate for the charge deficit related to the presence of the trivalent aluminum in the tetrahedrons. Also known to this art are certain rare zeolites wherein the silicon is replaced by tetravalent germanium. Similarly, trivalent elements, such as gallium and more rarely boron or beryllium, may be substituted for the aluminum.
In general, the composition of the zeolites may be represented by the overall formula: M.sub.2/n O; Y.sub.2 O.sub.3 ; xZO.sub.2, in dehydrated and calcined state. Z and Y respectively represent the tetravalent and trivalent elements of the TO.sub.4 tetrahedrons; M represents an electropositive element of valence n, such as alkali or alkaline earth metals, and constituting the compensating cations; x may range from 2 theoretically to infinity, in which case the zeolite is a crystallized silica.
Each type of zeolite has a distinct pore structure. The variation in the dimensions and in the form of the pores from one type to another is the reason for the differences in the respective adsorption properties thereof. Only those molecules having certain dimensions and shapes are able to enter the pores of a particular zeolite. In view of these remarkable properties, the zeolites are especially suitable for the purification or separation of gases or liquids, such as, for example, the separation of hydrocarbons by selective adsorption.
The chemical composition, in particular together with the nature of the elements present in the TO.sub.4 tetrahedrons and the nature of the exchangeable compensating cations, is also an important factor in the selectivity of adsorption and particularly the catalytic activity of these materials. They are used as catalysts or catalyst supports in the cracking, reforming and modifications of hydrocarbons and in the synthesis of numerous compounds.
Many zeolites exist in nature, i.e., the aluminosilicates, the availability and properties of which do not always satisfy the requirements of industrial applications. Consequently, the research and development effort for novel zeolites essentially of the aluminosilicate type has to date been considerable. Among the numerous examples of such effort, the following references are illustrative: zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite L (French Patent No. 1,224,154), zeolite T (French Patent No. 1,223,775), zeolite ZSM5 (U.S. Pat. No. 3,702,886), zeolite ZSM12 (U.S. Pat. No. 3,832,444), zeolite ZSM48 (EP 0,015,132).
Zeolites containing titanium in the TO.sub.4 tetrahedrons have also been proposed to this art. Compare French Patent No. 2,471,950, EP 104,107 and 100,119 and U.S. Pat. No. 3,329,481. However, the titanium bond in the crystalline system is of the octahedral type, rather than tetrahedral, the substitution of silicon by titanium in the structure of a zeolite being very difficult in the case of the aluminosilicates.
Zeolites are typically obtained from a reaction mixture which is converted in a hydrothermal medium, by a dissolution/recrystallization process, with the crystalline precipitate being calcined after separation and drying to yield an active zeolite.
The reaction mixture contains the elements T to be incorporated into the skeleton of the zeolite; these reagents generally are aqueous gels containing the oxides or hydroxides of the elements T.
The reaction mixture also contains a "mobilizer" promoting the dissolution of these reagents and their transfer from the aqueous phase into the zeolites under formation, and structural agents enabling formation of microporous spaces, together with the stabilization of the zeolite.
Hydroxide ions are used as the mobilizer. Thus, the reaction media generally have a pH higher than 10, on the one hand to insure the dissolution of the sources of silica and the other sources of the elements T, and, on the other, to facilitate the transfer of the soluble species into the zeolite in the process of formation.
Zeolites containing species easily soluble in a basic medium, such as, for example, aluminum, are well synthesized by this method.
However, it appears to be quite difficult to incorporate titanium in the TO.sub.4 skeleton of the zeolite, if the reaction medium is basic.
Furthermore, in a basic medium the metastable zeolites are obtained only if the reaction medium is super-saturated with active species. This gives rise to rapid nucleation resulting in small zeolite crystals, without the option of easily controlling the dimensions of such crystals.
In addition, syntheses employing basic reaction mixtures require the use of alkali or alkaline earth metal cations as compensating cations. These cations frequently must be subsequently eliminated, as they affect the catalytic or adsorbent properties of the zeolite. This elimination is typically carried out by repeated ion exchange using NH.sub.4.sup.+ cations. The zeolite containing ammonium cations is then calcined to eliminate them in the form of NH.sub.3.